Lecture series: SGL 201 – Principles of Mineralogy.
LECTURE 4
4.0 INTRODUCTION TO THE MICROSCOPIC STUDY OF MINERALS
4.1 LECTURE OUTLINE
Welcome to the fourth Lecture of this unit. You should now be familiar with the concepts of
optical mineralogy covered in Lecture 3. In this lecture, we are going to learn firstly about
the important components of the petrographic microscope itself. Technologically,
microscopes vary in their design, not only in their appearance but also in the positioning and
operation of the various essential components. These components are present in all
microscopes and will be described briefly in this lecture. Although dual purpose microscopes
incorporating both transmitted and reflected light options are now available in the market, its
is more convenient for the purpose of this unit and for your ease of understanding, to
describe the two techniques separately. Secondly, in learning about the microscopic study of
minerals, we shall give a systematic description of the characteristic optical properties
observed under plane- and crossed-polarized light. Finally we shall review the general
procedures used in the making of thin- and polished sections.
OBJECTIVES
At the end of this lecture, you should be able to:
• Describe and illustrate the use of all the major components of the transmitted light
polarizing microscope
• Give a systematic description of minerals in thin sections using transmitted light.
• Explain the optical properties observed under plane-polarized light and crossed
polars.
• Give examples of minerals with distinctive optical properties
• Describe the use of the Reflecting Microscope.
• Describe the general procedures of making thin- and polished-sections.
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Lecture series: SGL 201 – Principles of Mineralogy.
4.2 THE TRANSMITTED-LIGHT MICROSCOPE
The polarizing transmitted-light microscope, commonly known as petrographic microscope,
consists essentially of a light source, a sub-stage condenser and a stage to hold the specimen,
an objective, and an eyepiece (Figure 4.1). In addition to these, there is a device for
producing polarized light, termed the polarizer, a graduated rotating stage as opposed to a
fixed stage, and a second polarizing device termed the analyser. A brief description of these
major components is discussed here below.
4.2.1 The light source
In transmitted-light studies a lamp is commonly built into the microscope base (Figure 4.1).
The typical bulb used has a tungsten filament, which gives the field view a yellowish tint. A
blue filter can be inserted immediately above the light source to change the light colour to
that of daylight.
Figure 4.1. The transmitted-light polarizing microscope
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Lecture series: SGL 201 – Principles of Mineralogy.
In older microscopes the light source is quite separate from the microscope and is usually
contained in a hooded metal box to which can be added a blue glass screen for daylight
colored light. A small movable circular mirror, one side of which is flat and the other
concave, is attached to the base of the microscope barrel. The mirror is used to direct the
light through the rock thin section on the microscope stage, and the flat side of the mirror
should be used when a condenser (described below) is present.
4.2.2 The polariser
The assumption is that light consists of electromagnetic vibrations. These vibrations move
outwards in every direction from a point source of white light, such as a microscope bulb. A
polarizing film (the polariser) is held within a lens system located below the stage of the
microscope and this is usually inserted into the path. On passing through the polariser, the
light is polarized and now vibrates in a single plane. This is called plane-polarized light
(PPL). In most UK microscopes the polariser is oriented to give an E-W vibrating incident
light.
4.2.3 Substage diaphragms
One or two diaphragms may be located below the stage. The field diaphragm, often omitted
on simple student microscopes, is used to reduce the area of light entering the thin section,
and should be in focus at the same position as the thin section: it should be opened until it
just disappears from view. The aperture diaphragm is closed to increase resolution, it can be
seen when the Bertrand’s lens is inserted.
4.2.4 The condenser or convergent lens
A small circular lens (the condenser) is attached to a swivel bar, so that it can be inserted into
the optical train when required. It serves to direct a cone of light on the thin section and give
optimum resolution for the objectives (described below) used. The entire lens system below
the microscope stage, incl